Conventional photometers are poorly suited for measuring the absorption of transparent specimens which have a nonuniform or complicated outer form. It is known to utilize photometers equipped with an Ulbricht sphere for specimens which have an outer form suitable for photometric measurements but which scatter greatly.
An arrangement of this kind is described for example in an article by E. Wendland entitled "Messung der diffusen Reflexion mit der Ulbricht-Kugel" and published in GIT Fachz. Lab., Volume 9, 1988, pages 993 to 997. The arrangement includes a conventional single-beam spectral photometer having an accessory in the form of an integrating sphere which is generally known as an Ulbricht sphere. The inner surface of the Ulbricht sphere has a good reflectivity and reflects radiation incident thereon without a preferred direction. The Ulbricht sphere has two openings for measurements on transparent specimens. The specimen is mounted ahead of one opening and irradiated with monochromatic light which impinges virtually vertically on the specimen. The radiation leaves the specimen in a large angular range in the sphere because of the diffusion. This radiation is reflected so often from the good reflecting inner surface of the Ulbricht sphere until the radiation passes through the second opening of the sphere and impinges on a detector behind the sphere.
This known arrangement has the disadvantage that it is poorly suited or not at all suited for solid specimens such as cut diamonds and other precious stones which do not have two mutually parallel surfaces. Many other solid transparent specimens whose absorption or spectral absorption one wishes to measure do not have two surfaces which are at least approximately parallel to each other without processing and which are suitable for optical measurements.